1. Field of the Invention
The present invention relates generally to wireless communication, and in particular to vehicle-to-vehicle wireless communication in a frequency band shared with other types of wireless communication.
2. Description of the Related Art
Vehicle-to-vehicle communication is an emerging technology with numerous applications under study, including collision warning systems. Many of the systems being developed use a frequency band allocated for use under the Dedicated Short Range Communications (DSRC) standards. The same frequency band is also used for vehicle-to-roadside communications, including electronic toll collection.
In a vehicle-to-roadside DSRC system, the allocated frequency band is divided into a plurality of frequency channels. Interference between communications in mutually adjacent or overlapping wireless service areas is avoided by the assignment of different channels to different areas. Packet collisions in communications on the same channel by multiple vehicles in a single wireless service area are avoided by a time division multiple access (TDMA) system. The roadside base station in each service area designates the channels used and controls the vehicles' transmission timings. The mobile stations in the vehicles carry out a frequency selection process and a transition process specified in the DSRC standards, which enables them to communicate with the base station in each communication area on the designated channels.
Vehicle-to-vehicle communication systems avoid packet collisions by using a carrier sense multiple access (CSMA) transmission control system in which, before transmitting on a desired carrier frequency, a mobile station detects the desired carrier signal, observes the signals transmitted by other mobile stations on this carrier frequency, waits for their transmissions to cease, and then transmits its own signal.
Since vehicle-to-vehicle communication systems and vehicle-to-roadside communication systems use the same frequency band, when a vehicle communicating with another vehicle enters a vehicle-to-roadside communication service area, interference may occur. Disruption of vehicle-to-roadside communication by interference from vehicle-to-vehicle communication is particularly troublesome, because by the time the interference ends, the vehicle that is trying to communicate with the roadside base station may have left the base station's wireless service area.
In Japanese Patent Application Publication No. 2001-237847, Mizoguchi et al. describe a method of avoiding interference between a wireless communication system and, for example, a weather radar system using the same frequency band. A station in the wireless communication system receives both the radar signal and the desired communication signal, detects the levels of both signals, and refrains from transmitting communication packets for a predetermined time if either signal level exceeds a predetermined level. Besides preventing packet collisions, this scheme also prevents transmitted packets from interfering with the radar signal.
If this method were to be applied to vehicle-to-vehicle communications, however, then communication would be interrupted whenever interference from another system exceeded a predetermined level. Such interruptions could have serious consequences when the information to be transmitted by vehicle-to-vehicle communication is an emergency signal or warning signal.
Another problem in the system described by Mizoguchi et al. is the need for extra circuitry to identify or isolate the interfering signal so that its level can be measured.
Yet another problem is that the frequency of the interfering signals encountered in vehicle-to-vehicle communication systems may vary as the vehicles travel down the road, making it necessary to search for the interfering frequency by scanning the available channels, a time-consuming process.